feat: optional iframe init option for embedded checkout

[MHJaafar9869]

Summary

Adds an optional, unsigned iframe field to the checkout init request, mirroring the existing payment_mode field. When true (server reads it as boolean; true/1), it enables the embedded/iframe checkout experience. Only the paylink-core module is changed; the Spring Boot starter inherits automatically.

Like payment_mode, iframe is sent in the v2 init request body but excluded from the HMAC signature.

Changes

• CreateInvoiceParams.iframe(Boolean) builder method (after paymentMode(...)).
• FieldOrders.INVOICE_CREATE registers FieldSpec.unsigned("iframe") after payment_mode. Coerce.toWire already maps Boolean true->"1", false->"0".
• IframeOptionTest (JUnit 5): proves .iframe(true) yields "iframe"="1" (and false->"0") in the body built by SignedBody.build(...), and that the signature is identical to an otherwise-identical params object without iframe — proving the field is unsigned.
• README: embedded-checkout note/example under the quick start.
• CHANGELOG: new ## [0.2.0] section.
• Version bump 0.1.0 -> 0.2.0 across the root pom, both module parent references, the starter's paylink-core dependency, and README install snippets.

The shared golden-signatures.json fixture is intentionally not modified; the existing golden-signature parity tests continue to pass, which independently confirms iframe does not affect the signature.

Verification

Maven is not installed in this environment, so tests were run by compiling paylink-core (main + test) against Gson and JUnit 5 and executing them via the JUnit Platform Launcher.

[ 30 tests found      ]
[ 30 tests successful ]
[  0 tests failed     ]

All 30 core tests pass, including the golden-signature parity suite and the 3 new IframeOptionTest cases.